I found long ago, like Chrisu, that placing two entities at diagonal
corners of the "worldspace" I wish to work in did the trick.
Point-of-note I created these entities DYNAMICALLY in a level_load(NULL) level.
They werent even made MDL or physics entities, just NULL entities.
But beware, it was only very crude physics testing, so it may not work with
all the physics stuff. Back in about 7.10 I think it was.
Hope this is of some help.